Unfortunately, a lot of pet store employees lead customers down the wrong path with some of the impractical advice that they give - and sometimes, aside from being ridiculous, it can be completely wrong and detrimental to your fish's health. For instance, I've met many aquarists that were still under the impression that transferring "aged" water to a new fish tank will cycle it.
This is very far from the truth. In fact, all you're really transferring is water heavily contaminated with toxins, like ammonia. In order to cycle a tank you need to allow the beneficial bacteria to establish a colony - this takes around 6-8 weeks, sometimes longer depending on the situation. However, instead of using aged water you can use aged filter media or gravel from an already established, healthy tank.
Additionally, many aquarists, both new and experienced, are unaware that "aging" water for 24-48 hours does not remove all of the harmful toxins anymore. You need a quality tap water conditioner, regardless of whether you have left the water out to age or not. Aging the water will not remove chloramines, or any of the heavy metals that are present - both of which can kill your fish.
A word of advice? Always keep in mind that merely working at a pet store does not make someone an expert. You should always come prepared with your own research, and at the very least, cross check the advice that you received from the pet store employee.